By George Beck, Colorado State University Cooperative Extension, Weed
Specialist

Noxious weeds, AKA invasive
plants, are an insidious problem in Colorado and throughout western U.S.

Noxious weeds displace native plants
and disrupt evolved ecosystem processes. Infestations of noxious weeds in pastures,
rangelands, and other natural areas readily disperse onto adjacent land causing further
problems and eventually disperse into agronomic fields where they decrease crop quality
and yield. Weeds will spread  that is their nature.

Prevention is the most powerful form
of weed management and the cheapest and easiest weed to manage is the one you do not have.
One of the best ways to prevent weeds from spreading is to control existing infestations.
Fall is a good time to exert noxious weed control. Herbicides are the primary tool of
choice in the fall. Biological control agents generally have finished preying on weeds by
fall and most grazing livestock will not consume weeds this time of year.

Mowing and hand pulling are most
effective when practiced before weeds go to seed. Biennial weeds, such as musk thistle and
diffuse knapweed, are in the rosette growth stage (except those setting seed) and can be
readily controlled with herbicides.

Perennial weeds, such as Canada
thistle, Russian knapweed, and leafy spurge, also are very susceptible to fall-applied
herbicides. The physiology of these latter weeds changes as the day length continues to
shorten and temperatures decrease. It is not fully understood how these changes increase
their susceptibility to herbicides applied in fall, but gardeners can seize the
opportunity and take advantage of this fact.

The growing season is not finished,
so make your fall noxious weed management plans and take advantage of their susceptibility
to control methods this time of year.